1. Field of the Invention
The present invention relates to a projector, and more particularly, to a projector having a tilt angle measuring device for calculating the tilt angle of the optical axis of projection of the projection device of the projector and the projection surface.
2. Description of the Related Art
The rapid progress of liquid crystal technology and DLP (registered trademark) (Digital Light Processing) technology has been accompanied by advances in the development of more compact and more highly functional projectors. With this development, the use of projectors of the prior art having the purpose of image projection has expanded, and projectors are now receiving attention as large-sized display devices for taking the place of display-type televisions for home use.
However, in contrast with a display-type television, the image plane of a projector may be a screen or a wall, and the problem therefore arises of distortion in the image that results from the relation between the axis of projection of the projector and the projection surface. To provide a solution to this problem, Japanese Patent Laid-Open Publication No. 281597/97 discloses a method that includes a detection means for detecting the installation angle of the liquid crystal projector and a distance detection means for detecting the distance between the liquid crystal projector and the object of projection whereby the angle of the liquid crystal display unit is adjusted according to the angle that is calculated from the two detection results. In addition, Japanese Patent Laid-Open Publication No. 169211/01 discloses a distortion correction method in which: the light spot of a laser pointer for which the angle can be controlled is projected onto a curved screen; a dot image for measurement is generated and projected onto the screen from a projector, the image of the light spot and dot image is captured by a camera and their positions measured; and the dot image is shifted until the two points coincide, whereupon the picture element coordinates of the dot image in a frame memory are converted to the coordinates of the light spot on the input image and set in a coordinate conversion parameter memory. A liquid crystal projector is also disclosed in Japanese Patent Laid-Open Publication No. 197249/97 in which: an image pattern that is projected onto a screen is passed through a slit and received by a line sensor, whereupon the position of the reflected light of this image pattern is used to find the distance to the screen; the image pattern that is projected onto the screen is shifted on the screen and the tilt of the screen is found based on the shift of the position of the reflected light of the image pattern that is passed through the slit and received by the line sensor; and diagonal adjustment is realized by controlling a diagonal actuator to tilt the projector unit backward and forward.
Techniques have already been put into practical use whereby, if the tilt of a screen in perpendicular and horizontal directions with respect to the axis of projection of a projector is known, an image that is free of distortion can be projected onto a screen by converting coordinates in the frame memory of the projector. Thus, Japanese Patent Laid-Open Publication No. 5278/03 already discloses a projector in which, on the premise that the screen is installed perpendicularly, the tilt of the projector is detected by means of a gravity sensor, and the tilt in the perpendicular direction, which in particular tends to produce distortion, is measured and distortion then corrected in accordance with this tilt.
However, the method that is disclosed in Japanese Patent Laid-Open Publication No. 281597/97 requires mechanical adjustment of the angle of the liquid crystal display unit. The method disclosed in Japanese Patent Laid-Open Publication No. 169211/01 requires control of the angle of the laser pointer and therefore entails a complex structure. The method that is disclosed in Japanese Patent Laid-Open Publication No. 197249/97 requires shifting of the image pattern on the screen to detect the angle, and therefore cannot obtain the detection speed necessary for realizing distortion correction in real time. The method that is disclosed in Japanese Patent Laid-Open Publication No. 5278/03 assumes the perpendicular installation of the screen and therefore cannot provide accurate correction of distortion when the screen is not installed perpendicularly or when the screen is tilted in the horizontal direction with respect to the axis of projection of the projector.